Hitherto, to steel materials such as a zinc-plated steel sheet, an aluminum-plated steel sheet and the like, a rust prevention treatment by chromate, which uses hexavalent chromate, has been widely applied, and theses steel materials have been coated with an organic resin and then top coated with various coating compositions in order to provide an anti-fingerprint property and an anti-scratch property as required.
In recent years, there are moves to regulate or prohibit chromate treatments which hitherto have been applied to steel materials by a law against the background of increasing environmental issues. Since a chromate treated layer in itself has a high degree of corrosion resistance and an adhesion property to a paint, if this chromate treatment cannot be performed, these properties will be significantly deteriorated. Accordingly, it has been desired to form a rust preventing layer having good corrosion resistance and a good adhesion property to a paint without carrying out a chromate treatment.
In Japanese Kokai Publication Hei11-71536, there is described a coated steel sheet treated with a rust prevention treatment agent for metal surfaces in which silica and an epoxy compound are contained in a reaction product of an ionomer resin neutralized with a bivalent metal and an epoxy compound. This coated steel sheet provides an improvement in adhesion to curable resins such as coating compositions. However, a coat of this coated steel sheet has low adhesion to metal material and particularly under the wet condition, water permeates through the coat and reaches the interface between the coat and the metal material and therefore the coat is peeled off.
In Japanese Kokai Publication 2000-273659, there is described a coated steel sheet treated with a rust prevention treatment agent for metal surfaces in which silica, an epoxy compound, a silane coupling agent and thiosulfate ion are mixed in a reaction product of an ionomer resin neutralized with a monovalent metal and a bivalent metal, a polyolefin resin neutralized with amine and an epoxy compound. In such a coated steel sheet, alkali resistance and an adhesion property to a paint are more improved than that in the coated steel sheet in Japanese Kokai Publication Hei11-71536 according to how much the silane coupling agent are post-added. But, the adhesion to metal material is insufficient.
In Japanese Kokai Publication 2003-155451, a coated steel sheet treated with a water-borne coating agent into which a water-dispersible resin, silica particles and an organic titanium compound are mixed is described. However, this coated steel sheet may be insufficient in properties of an adhesion property to a substrate and press oil resistance of a coat, and a coated steel sheet, performance of which is further improved, is required.
In Japanese Kokai Publication 2005-281863, a coated steel sheet, which is provided with a coat containing a crosslinked resin matrix and an inorganic rust preventive agent, is described.